I scrub new to me used kegs the first time I clean them- also replace all the O rings. After that, when one kicks, I pop the top that day or the next, rinse with hot tap water, then fill with hot homemade PBW for an overnight soak. Next day dump, rinse twice, fill with StarSani for at least a day before pushing into the next keg. Leave it pressurized till time to fill.
If a new from factory I just do the StarSani
If a new from factory I just do the StarSani